1. Field of the Invention
The present invention relates to semiconductor integrated circuit devices, optical scanners using semiconductor integrated circuit devices, image forming apparatuses using optical scanners, and methods of identifying return light.
2. Description of the Related Art
Conventionally, in image forming apparatuses that form electrostatic images by exposing a photosensitive body to laser light emitted from a laser diode of an optical scanner, an initialization operation is performed to determine a current value for causing the laser diode to emit light. According to this initialization operation, the value of a current caused to flow through the laser diode, or the amount of light emission of the laser diode, is caused to increase gradually, and the gradually-increasing amount of light is detected with a photodetector provided inside a laser unit. When the photodetector-detected value becomes a predetermined value, a corresponding current value is determined as a value for causing the laser diode to emit light.
A more detailed description is given below with reference to corresponding drawings.
FIG. 1 is a graph illustrating the relationship between the drive current and the amount of light (light amount) of a common laser diode. In FIG. 1, the horizontal axis represents the drive current of the laser diode, and the vertical axis represents the amount of the laser light emitted from the laser diode.
As illustrated in FIG. 1, the laser diode (hereinafter referred to as “LD”) starts to emit light in response to the drive current exceeding a threshold. The drive current at this point is referred to as a threshold current Ith. The drive current enters a light emission region when exceeding the threshold current Ith. In the light emission region, a light emission current Iη is proportional to the light amount. In FIG. 1, IthN denotes a threshold current at normal temperature, IηN denotes a light emission current for a light amount L at normal temperature, IopN denotes a drive current for the light amount L at normal temperature, IthH denotes a threshold current at high temperature, IηH denotes a light emission current for the light amount L at high temperature, and IopH denotes a drive current for the light amount L at high temperature.
As illustrated in FIG. 1, the threshold current Ith and the differential efficiency η (the slope of the straight line in the light emission region in FIG. 1) vary relative to temperature. The differential efficiency η tends to decrease with high temperatures. Accordingly, the drive current Iop for maintaining a constant amount of light (the light amount L) varies according to temperature (IopN<IopH), so that the drive current Iop for a desired amount of light (the light amount L) is corrected in an application using the LD. A circuit that automatically corrects (controls) the drive current Iop so that the light amount is constant (at L) is referred to as “automatic power control (APC) circuit.” All or some of the circuit blocks of the APC circuit are implemented as a semiconductor integrated circuit device.
FIG. 2 is a diagram illustrating a configuration of an APC circuit mounted in a conventional image forming apparatus.
Referring to FIG. 2, an APC circuit 200 includes a current generation part 202, an I/V conversion part 203, a comparison part 204, and a current control part 205. The APC circuit 200 is connected to a laser unit 201. The laser unit 201 includes a laser diode 201a (hereinafter referred to as “LD 201a”) and a photodetector 201b (hereinafter referred to as “PD 201b”).
Referring to FIG. 2, when the drive current Iop is caused to flow from the current generation part 202 to the LD 201a of the laser unit 201, the LD 201a emits laser light proportional in amount to the drive current Iop in the light emission region. The laser light is emitted not only toward an object (target) of light emission but also in the direction of the PD 201b of the laser unit 201. The PD 201b generates a monitor current Im proportional to the amount of the emitted light. The monitor current Im is converted into a monitor voltage Vm by the I/V conversion part 203. The monitor voltage Vm indicates the amount of the laser light emitted by the LD 201a. 
The comparator 204 compares a reference voltage (reference light amount) Vref and the monitor voltage Vm, and outputs the result of the comparison to the current control part 205. The current control part 205 controls the current of the current generation part 202 based on the comparison result output by the comparator 204. The amount of the laser light emitted by the LD 201a is thus maintained at a desired value corresponding to the reference voltage (reference light amount) Vref.
In this configuration, at the time of causing the LD 201a to emit light, the above-described initialization operation first sets the drive current lop to a value less than or equal to the threshold current Ith; thereafter gradually increases the drive current Iop; detects the amount of light at the time with the PD 201b; and when the detection value of the PD 201b becomes a predetermined value, determines a corresponding current value as a value for causing the LD 201a to emit light. The determined current value is maintained for a predetermined period of time, and after passage of the predetermined period of time, the current value is redetermined in the same manner.
In order to perform the initialization operation properly, it is desirable to detect the monitor current Im with accuracy. If light that is not supposed to be detected is detected in the PD 201b, the amount of the laser light emitted by the LD 201a varies, for example, becomes excessively large or small, and is not maintained at a desired value. In this case, the LD 201a continues emitting light for a predetermined period of time at a value different from the desired value. Accordingly, optical writing may not be performed with accuracy in the application, and in the worst case, the drive current Iop may exceed a permissible value to break the LD 201a. (See, for example, Japanese Laid-Open Patent Application No. 2001-264011, Japanese Laid-Open Patent Application No. 2003-092453, Japanese Laid-Open Patent Application No. 2007-200513, Japanese Laid-Open Patent Application No. 5-191603, Japanese Laid-Open Patent Application No. 5-243652, Japanese Laid-Open Patent Application No. 11-298075, Japanese Laid-Open Patent Application No. 11-273120, and Japanese Laid-Open Patent Application No. 63-124971.)